1. Field of the Invention
The present invention relates to scale, corrosion and algae control in cooling towers and, more particularly, to a system and method of continuously treating cooling tower waters.
2. Description of the Prior Art
Water is the closest thing to a universal solvent known. In its pure condition (without dissolved materials) it is very corrosive. Most normally used industrial metals will gradually corrode or dissolve in distilled water. Some of the dissolved materials, like carbon dioxide, make water even more aggressive, both to metals and to non-metals. This solvent, water, which is a universal (and cheap) heat transfer medium, has a specific heat of 1 Btu/lb/.degree.F. Water with its tremendous capacity to absorb and store heat, can absorb 33 times more heat than lead and 10 times more than iron.
All water utilized as the heat transfer medium in cooling towers has traditionally required some form of treatment. In the past chromium salts, generally in an acid condition, have been extensively used. Cooling tower water treatment implies that the waters are passivated in such a way that the following major problems do not develop or are greatly reduced:
(1) Scale build-up to the point of equipment or function failure. PA1 (2) Corrosion of metals or degradation of wood to the point of function failure. PA1 (3) Biological contamination to the point of function failure.
Airborne particles carry with them microorganisms which, after coming in contact with the cooling water, are distributed throughout the cooling water system to form algae, slime, and bacterial growths. Algae growth can accumulate to the point at which it interferes with the water flow in the cooling system. Slime in condensers restricts the water flow and acts as an insulator reducing the cooling capacity. This results in higher head pressures and increased load on circulating pump motors with corresponding increases in power consumption. Accumulations of organic matter can cause localized corrosive attack, resulting in premature equipment failure. Fungus action can cause wood destruction in towers.
These growths have been controlled with ozone. The ozone is dissolved in the cooling water where it attacks the bacteria and microorganisms and destroys them by oxidation. However, the ozone level is very low of the order of 0.2 mg/l of ozone and the ozone has always been used in conjunction with other chemical agents or electrochemical or electromechanical controls.
Corrosion is a destructive attack on metals which may be chemical or electrochemical in nature. Direct chemical corrosion is limited to unusual conditions involving highly corrosive environments or high temperature or both. Examples are metals in contact with acids or alkalies. However, a large part of the phenomena involving the corrosion of metals submerged in water, is electrochemical in nature. In cooling water systems, electrolytes are always present--namely ions dissolved in the water. Electrodes (anode and cathode) are also present. These electrodes may be two different metals, such as copper condenser tubes and steel condenser heads of the shell, or different areas of the same metal. In dissimilar metal corrosion, the less noble metal (anode) through corrosion will proceed to the more noble metal (cathode) (called galvanic corrosion). Corrosion of the same metal surface occurs if a difference in potential exists between two areas of the metal. One will act as the anode and the other as the cathode. Disintegration of metal will occur only at the anode. However, anode and cathode areas may shift periodically, resulting in uniform corrosion instead of pitting.
Past use of the chromium treatment did little to prevent biological problems from developing in cooling towers; however, as an inhibitor, it was very effective (and cheap) in protecting metals from corrosion. The acid condition of the treatment was also conducive to a reduced tendency for biological growth or scale development. Recent restrictions by the Environmental Protection Agency (EPA), on the use of chromium materials has made the search for alternative treatment materials necessary.
While the combined acid chromate treatment has long been the dominant technique for treating cooling tower waters, the non-chromate alkaline treatments are gaining popularity (by necessity). In some ways the modern alkaline type treatment resembles that which previously was known as the threshold treatment. This was a use condition where polyphosphates and selected organic materials were used in relatively low concentrations (threshold limit) to produce a partial sequestration of the calcium carbonates. Scale forming materials were prevented from giving hard scales, but they did allow a soft mud to accumulate in the reservoirs of the system. The new formulations that are being offered for this purpose are more sophisticated and higher in costs. They require much more supervision, monitoring, and control.